Just saw that the transaction costs for m-pesa are quite high - the company makes ~20% profit

The transaction costs listed on the wikipedia page you cite aren't trivial, but would average well less than 20% unless most transactions are (a) very small and (b) to unregistered users. I'm missing something.

EDIT: could it just be that their profit is 20% of expenses, as opposed to 20% of the money that flows through the M-Pesa network?

maybe using crypocurrency

That article doesn't really show that cryptocurrency helps here. Mostly they're unhappy with transaction fees on international remittances, but you can have low transaction fees just by automating interactions with the money transfer organization, without going to cryptocurrency. And with cryptocurrency generally you pay someone a fee to get your money into the cryptocurrency and then your recipient pays someone else a fee to get it into their local currency.